Resin-molded semiconductor device and a process for manufacturing the same

ABSTRACT

A process for manufacturing resin-molded semiconductor devices which are sealed in an epoxy resin is carried out without employing a metal mold. In the process, an epoxy resin is deposited on a sub-assembly consisting of a semiconductor pellet brazed to axial leads while turning the sub-assembly with the axial leads as a center so that the epoxy resin is applied to a predetermined shape; then the epoxy resin is heated at an elevated temperature while turning the sub-assembly so that surface portions of the epoxy resin are hardened and the epoxy resin is further heated at an elevated temperature without turning the sub-assembly so that the epoxy resin is completely hardened.

BACKGROUND OF THE INVENTION

The present invention relates to a resin-molded semiconductor device,and more specifically to a resin-molded axial lead-type semiconductordevice.

Semiconductor devices have been extensively used which are obtained bysealing in an epoxy resin a sub-assembly in which a semiconductor pelletis held by brazing between a pair of axial leads. So far, however, thesub-assembly is sealed in the epoxy resin by using a metal mold. When aglass is to be used for the purpose of sealing, a slurry of glass isapplied round a semiconductor pellet in the sub-assembly and is thenbaked. In this case, there is no need of employing the metal mold. Whenthe epoxy resin is used, however, the metal mold must be employed unlikethe case of using the glass. The reason for requiring the metal mold isbecause there is no one-can-type epoxy resin available which cures tosome extent within a few minutes. Namely, the resin initially istransformed into a sol just before the resin cures, and its viscositydecreases strikingly relative to its viscosity at ordinary temperature.Accordingly, the resin separates from the sub-assembly.

Japanese Patent Laid-Open No. 1174/1980 discloses a method according towhich the sub-assembly is sealed in a glass, and the epoxy resin isfurther applied thereto. In this case, however, the applied epoxy resinforms a film which is not thick enough to achieve the desired sealing.

According to a sealing method which is now practiced by using an epoxyresin, it is necessary to employ a metal mold. Therefore, cumbersomeoperation is required for setting the sub-assembly within the metalmold, for injecting the epoxy resin therein, for parting thesub-assembly from the metal mold, and the like. In particular, it isdifficult to continuously perform the setting and parting operations,thereby imposing a bottleneck for mass-production. When a parting agentis contained in the epoxy resin to facilitate the parting operation, theadhesiveness between the resin and the leads is reduced, and gaps areoften formed between the leads and the sealing layer in thesemiconductor device. Water infiltrates through the gaps and reaches thesemiconductor pellet; i.e., the semiconductor device loses desiredcharacteristics and further loses reliability in operation.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a resin-moldedsemiconductor device sealed in an epoxy resin which has high quality andreliability.

Another object of the present invention is to provide a process formanufacturing resin-molded semiconductor devices which are sealed in theepoxy resin without employing a metal mold.

The present invention relates to a process which comprises:

a step of horizontally holding a sub-assembly that consists of asemiconductor pellet brazed to axial leads, and of depositing an epoxyresin of a predetermined amount onto the semiconductor pellet whileturning the sub-assembly with the axial leads as a center, so that theepoxy resin is applied to a predetermined shape;

a step of heating the epoxy resin at a temperature of 160° to 180° for aperiod of 2 to 10 minutes while turning the sub-assembly, so thatsurface portions of the epoxy resin are hardened; and

a step of heating the epoxy resin at a temperature of 160° to 200° C.for a period of 3 to 24 hours without turning the sub-assembly, so thatthe epoxy resin is completely hardened.

The invention also relates to a semiconductor device which is obtainedby the above-mentioned process. The invention can be put into practiceby using an epoxy resin which has a suitable hardening property and asuitable thixotropic index.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a vertical section view of a resin-molded semiconductor deviceaccording to an embodiment of the present invention;

FIG. 2 is a vertical section view of a resin-molded semiconductor deviceaccording to another embodiment of the present invention;

FIG. 3 is a vertical section view of a resin-molded semiconductor deviceaccording to a further embodiment of the present invention; and

FIG. 4 is a vertical section view of a resin-molded semiconductor deviceaccording to yet further embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a vertical section view of a resin-molded semiconductor deviceaccording to the present invention. In FIG. 1, a silicon pellet 1 havinga pn junction J has nickel-plated layers 2 on both of its surfaces sothat soldering can be easily accomplished. The silicon pellet 1 is heldbetween a pair of copper header leads 4 by soldering as designated byreference numeral 3. Silver-plated layers 5 are formed on the surfacesof the header leads 4. To stabilize the surfaces of pn junction J of thesilicon pellet 1 of the thus constructed sub-assembly, a conventionalsurface-stabilizing material 6 such as silicone resin is provided aroundthe side periphery of the silicon pellet 1, and then a sealing layer isformed by using an epoxy resin.

Specifically, in the embodiments of the invention there is used aone-can epoxy resin which has epoxy groups in the molecules thereof, andwhich contains 3 to 15 moles of an organic dibasic acid dihydrazide per100 moles of the epoxy resin, and 2 to 7 moles of an imidazole compoundrepresented by the following formula; ##STR1## wherein R₁ is a methylgroup, a hydrogen atom or a hydroxymethyl group, and

R₂ is an alkyl group, per 100 moles of the epoxy resin.

It is also allowable to use a one-can epoxy resin which does not containthe organic dibasic acid dihydrazide, but which contains only theimidazole compound represented by the above formula in an amount of 2 to20 moles per 100 moles of the epoxy resin.

This resin exhibits a thixotropic index and hardening property that aresuited for effecting the manufacturing process that will be describedbelow. This resin exhibits a viscosity ratio of 1.0 to 2.5 as measuredby a rotation viscometer under the conditions of 4 turns a minute and 20turns a minute, and exhibits a gellation time of 0.8 to 3 minutes in aconstant temperature bath.

By using this resin, the device of the embodiment of FIG. 1 ismanufactured as described below.

First, the epoxy resin of a predetermined amount is dropped onto thesub-assembly which is contained in the surface-stabilizing material 6and which is rotated at a speed of 2 turns a second with the electrodes4 being held nearly horizontally. The resin will be required in anamount of about 0.05 grams when it is to be applied to a length of 4 mmand to a diameter of 3.5 mm onto the sub-assembly in which the leadshave a diameter of 0.6 mm and the header portions 12a have a diameter of1.5 mm. As the sub-assembly rotates, the epoxy resin builds up in ashape as shown. That is, in the portions where the epoxy resin comesinto contact with the electrodes 4 or the surface-stabilizing material6, the resin transforms into a sol due to the frictional force producedby the rotation, whereby the viscosity decreases and the resin isapplied well onto the header leads 4 and the surface-stabilizingmaterial 6. In the portions remote from the sub-assembly, on the otherhand, the frictional force decreases and the centrifugal force onlyacts. Therefore, the epoxy resin is not transformed into sol, andmaintains the shape as shown. Then, the surface only of the epoxy resinis hardening by heating while rotating the sub-assembly. Good resultswill be obtained if the epoxy resin is heated at 160° to 180° C. for 2to 10 minutes. In this case, the epoxy resin in the surface is oncesolated by the heat and its viscosity decreases conspicuously. However,the internal epoxy resin is not solated and has a viscosity which isgreater than that of the epoxy resin in the surface. Therefore, theillustrated shape is roughly maintained, and the surface portion only ishardened. The internal epoxy resin is confined by the surface portionwhich is hardened. At this moment, therefore, the epoxy resin does notfall from the sub-assembly although it is not rotated; i.e., the epoxyresin which is applied thereto maintains its shape. Therefore, there isno need of rotating the sub-assembly. That is, rotation of thesub-assembly is stopped, and the epoxy resin is heated at 160° to 200°for 3 to 24 hours so that it is perfectly cured up to its internalportion, at a different place as required. In the above-mentionedembodiment, the epoxy resin is hardened by heating. The epoxy resin,however, can also be hardened by the conventional methods such asirradiation with ultraviolet rays, X-rays, infrared rays, and electronbeam.

According to this process as described above, the sub-assembly can bemolded without the need of using the metal mold. Therefore, the numberof manufacturing steps can be reduced, and the manufacturing operationcan be continuously carried out using an automatic machine tomass-produce devices maintaining stable quality.

The epoxy resin is hardened after it is sufficiently applied to theheader leads 4 or the surface-stabilizing material 6. Therefore, thesealing layer 11 is well adhered onto the header leads 4 or thesurface-stabilizing material 6. Further, since the epoxy resin does notcontain a parting agent, no gap develops that occurred in theconventional procedure between the leads and the sealing layer, andthere is obtained a semiconductor device having an increased resistanceagainst moisture.

FIG. 2 illustrates another embodiment of the present invention, in whichthe portions same as, or corresponding to, those of FIG. 1 are denotedby the same reference numerals.

This embodiment employs double header leads 12 which have flangeportions 12b at positions separated from the header portions 12a.

The flange portions 12b block the epoxy resin from flowing in the axialdirection when it is being applied, work to trim the shape of thesealing layer 11, and contribute to stabilizing the quality.

FIG. 3 shows a further embodiment of the present invention, in which theportions same as, or corresponding to, those of FIG. 1 are denoted bythe same reference numerals.

In this embodiment, the surface-stabilizing material is not used thatwas employed in the embodiment of FIG. 1, and the sub-assembly isdirectly wrapped in the sealing layer 11 composed of the epoxy resin.

The above-mentioned epoxy resin does not contain mobile ions such asNa⁺, Cl⁻, or the like, that make the exposed surfaces of pn junction Jof the silicon pellet 1 unstable. Therefore, use of thesurface-stabilizing material can be eliminated so far as the epoxy resinis not contaminated by mobile ions such as Na⁺, Cl⁻, or the like. Thishelps further reduce the number of manufacturing steps.

FIG. 4 shows still further embodiment of the present invention, in whichthe portions same as, or corresponding to, those of FIG. 1 are denotedby the same reference numerals.

In this embodiment, a plurality of silicon pellets 1a, 1b, . . . , 1nhaving pn junctions (not shown) are laminated in a predeterminedrectifying direction, adhered by brazing as designated at 13, and arethen supported between the header leads 4 by brazing as designated at 3.Here, the brazing material 3 has a melting point lower than that of thebrazing material 13, such that the laminate of silicon pellets 1a, 1b, .. . , 1n is not deformed when it is being brazed to the header leads 4with brazing material 3. FIG. 4 does not show nickel-plated layersformed on the silicon pellets 1a, 1b, . . . , 1n.

There will be obtained a withstand voltage which increases in proportionto the number of silicon pellets 1a, 1b, . . . , 1n.

According to the process of the present invention, a sub-assembly can bemolded sufficiently even if there is a large distance between the twoheader leads 4.

According to the present invention as described above, it is possible toobtain a resin-molded semiconductor device which can be easily molded ina continuous manner, which is suited for being mass-produced, and whichmaintains high quality and reliability.

What is claimed is:
 1. A process for manufacturing semiconductor devicescomprising:a step of horizontally holding a sub-assembly that consistsof a semiconductor pellet brazed to a pair of axial leads, and ofdepositing an epoxy resin of a predetermined amount onto thesemiconductor pellet while turning the sub-assembly with the axial leadsas a center; a step of heating the epoxy resin at a temperature of 160°to 180° C. for a period of 2 to 10 minutes while turning thesub-assembly, so that surface portions of the epoxy resin are hardened;and a step of heating the epoxy resin at a temperature of 160° to 200°C. for a period of 3 to 25 hours while maintaining the sub-assemblystationary without turning it, so that the epoxy resin is completelyhardened.
 2. A process for manufacturing semiconductor devices accordingto claim 1, wherein the epoxy resin which is to be deposited has athixotropic index of 1.0 to 2.5, and a gelation time of 0.8 to 3minutes.
 3. A process for manufacturing semiconductor devices accordingto claim 2, wherein the epoxy resin which is deposited is a one-canepoxy resin which has epoxy groups in the molecules, and which contains3 to 15 moles of an organic dibasic acid dihydrazide per 100 moles ofthe epoxy resin, and 2 to 7 moles of an imidazole compound representedby the following formula; ##STR2## wherein R₁ is a methyl group, ahydrogen atom or a hydroxymethyl group, andR₂ is an alkyl group, per 100moles of the epoxy resin.
 4. A process for manufacturing semiconductordevices according to claim 2, wherein the epoxy resin which is depositedis a one-can epoxy resin which has epoxy groups in the molecules, andwhich contains 2 to 20 moles of the imidazole compound represented bythe following formula; ##STR3## wherein R₁ is a methyl group, a hydrogenatom or a hydroxymethyl group, andR₂ is an alkyl group, per 100 moles ofthe epoxy resin.